The effects of body size and sexual dimorphism on the reproductive behaviour of socke

Abstract. Factors associated with the evolution of sexual dimorphism in sockeye salmon were examined by relating individual behavioural correlates of reproductive success with morphological measurements in both sexes. Adult salmon were captured, measured and tagged, then released and observed in Iliamna Lake, Alaska. Males were generally larger than females (447 versus 428 mm) and had larger dorsal humps and more elongated jaws for their length. Indices of male reproductive success varied considerably among individuals (e.g. 0-12 observations in dominant status). Male social status was positively associated with both length and dorsal hump size, independent of length. All resident females completed spawning and only nine of 82 had their nests dug up by other females. However, size-related variation in spawning-site location was observed; larger females tended to spawn in shallower water than did smaller females. Thus reproductive success apparently varied much more in males than in females. Consistent with the theory regarding the evolution of sexual dimorphism, the greater variability in male reproduction was directly associated with the greater average size and exaggerated shape of males.